"Add Action Strip" in NL animation panel is introducing mystery motion
The original armature and the problematic one aren't identical: the thigh bones have a different pivot point.
In this kind of rigs (pure FK) only the hip bone has loc infos, all the others bones are driven by rotations, and by parent-child relations.
If you move the pivot point of thoose rotations, the animation will start to be screwed up.
To solve, move in edit mode the heads of the thigh bones up on Z axis until they return to their original location (if this is compatible with your character)
If this is not possible, you have to either manually adjust the animation or (better) create an IK chain to lock the feet movements.